1. Field of the Invention
2. Related Technology
The present invention relates to droplet deposition apparatus and in particular to ink jet printers
Ink jet printers are no longer viewed simply as office printers, their versatility means that they are now used in digital presses and other industrial markets. It is not uncommon for print heads to contain in excess of 500 nozzles and it is anticipated that “page wide” print heads containing over 2000 nozzles will be commercially available in the near future.
It has been found that circulating ink through the print head when printing and when not printing has a beneficial affect on the droplet characteristics since the temperature may be controlled by a heat exchanger positioned outside the head.
A further improvement taught in W000/38928 is to continually pass ink through the ejection chambers. This improves the reliability of the print head by, at high enough flow-rates, reducing the possibility of air or dirt lodging in the nozzle and continually supplying fresh ink to the ejection chambers.
Because of the size of these large “page wide” print heads a large amount of ink is ejected from the heads when printing full black, i.e. when all the ejection chambers are printing at their maximum rate. It is proposed in print heads of the prior art that a flow rate of around ten times the maximum printing rate is used in order to help flush dirt out of the print head and maintain the head at a constant temperature.
Each nozzle should be at a similar pressure, preferably just below atmospheric, to minimise variations in ejection characteristics along the length of the print head.
Ink is supplied to the ejection chambers from elongate inlet and outlet manifolds that extend the length of the array and the pressure drop along the manifolds is a function of the circulation rate, manifold size and ink characteristics.
To maintain a constant pressure at each nozzle it is necessary, in view of the large flow of ink through the head, to provide inlet and outlet manifolds having large hydraulic diameters.
Print heads typically have nozzles arranged in linear arrays and are often grouped together in a printing machine such that the linear arrays of each print head lie parallel. In this arrangement multicolour printing is possible from a single pass of the paper under the print heads. A variation in the movement of the paper has one of the largest effects on drop landing position of droplets ejected from a print head possibly giving rise to visible defects in the printed image.
The effects of the variation in substrate movement can be reduced by locating the print heads close together. However, the large hydraulic diameters of the inlet and outlet manifolds often preclude this.
Ink is an expensive commodity and where the ink is a high value fluid such as, for example, biological fluid or fluid used to manufacture electronic component, the volume of ejection fluid contained within the large manifold may be prohibitive to the economic validity of the print head.